Gravity dependence at the bottom of the main sequence

We investigate the effects of gravity on the infrared spectra of objects around the M dwarf to brown dwarf transition. We focus on observations of the very low-mass objects TVLM 513-46546 and GJ 569B from 1 to 2.5 mu m. These objects have very similar spectral types and colours but they differ by more than a magnitude in luminosity; this indicates that their surface gravities differ by around 0.5 dex. We compare their spectra and present line identifications in the infrared. We investigate at low resolution the sensitivity of some of the atomic features to changes in surface gravities and make comparisons with recent atmospheric models. We identify seven surface gravity sensitive features. We find that the difference in surface gravity between the spectra are consistent with GJ 569B having a lower surface gravity than TVLM by at least 0.5 dex which suggests GJ 569B is a brown dwarf. Because of the relatively few surface gravity features which can be identified at low resolution, confirmation of this result should be made with observations at higher resolution which would enable more gravity sensitive features to be identified with better precision.